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Correction for Inertia.


Finally, expanding from P5 to PR in L. P. cylinder and re-

PR = 38 x

i'5 + '3 + '61
i'5 + '3



the  residual pressure.

While a small receiver should be adopted in Case II., a very
large one is advisable in Case III. in order to equalise the work
in the H. P. and L. P. diagrams. Of course, Case II. compels
a large gap in the combined diagram, on account of drop in
receiver and low-pressure cylinder, and the arrangement is not,
therefore, counselled. The student should compare actual
diagrams with ideal ones, and endeavour to distinguish between
Cases I. and III.

Correction of Indicator Diagram for Inertia.—The

indicator diagram, as obtained from the cylinder, does no more
than transcribe the changing pressure and volume on one or other
side of the piston. The actual pressures tending to move the
piston are not correctly shewn, at least not without a small
correction; but those transmitted to the crank, which are what
we most require to know, are considerably different, on account
of the deductions and additions required to respectively start
and stop the reciprocating parts at the beginning and end
of each stroke. We shall now examine the modifications to be
made in the indicator diagram in order to arrive at the tangential
pressure on the crank pin; and, to make the investigation as
useful as possible, shall take an actual case of a vertical engine,
where there is not only the inertia force to contend with, but
the dead weight of the moving mass. In a horizontal engine
there is no such dead weight, while in a diagonal engine the
pressure along the incline caused by the weight is the effective

Let the crank circle, j K LM, Fig. 660, have a radius of i' 9",
as measured by its own scale. Divide the circumference into,
say, 20 equal parts, and, with a connecting rod 7' 6" long, mark
corresponding positions of piston stroke from A to B. Draw the
polar curves, KU and UM, by the method given at p. 491, and
transfer the ordmates to the base A B, so as to form the velocity